Traditional Manufacturing Techniques versus Mechanosynthesis. The benefits of being able to manufacture with microscopic precision are well-known. For example, lithography is used to create the features on integrated circuits and may also be used to create MEMS (micro-electromechanical systems) or NEMS (nano-electromechanical systems) devices. Smaller features on integrated circuits enable them to run faster and use less power, and MEMS and NEMS technologies are used to create devices as diverse as airbags and cell phones (e.g., accelerometers and attitude sensing), projection screens (e.g., digital light projection), and medical diagnostics (e.g., lab-on-a-chip devices).
However, even though such devices or the features on such devices may be microscopic, they are not atomically-precise nor are they of the scale of single atoms. For example, the smallest feature size planned for use by Intel Corporation in 2013 is 14 nanometers, in their “Broadwell” line of processors. This is about 90 times the diameter of a carbon atom, and well over 100 times the diameter of a hydrogen atom.
The precision of lithography continues to be improved, and various other technologies are being pursued in an attempt to manufacture ever-smaller features and devices. For example, self-assembly is aimed at using microscopic units that spontaneously assemble to create tiny features or structures. But self-assembly is limited in the structures that can be created because of the need to design around the shape, charge, solubility or other requirements of the individual units.
Many other techniques for the creation of microscopic features and devices also exist. For example, e-beam deposition, micro-machining, and selective etching can all be used to create microscopic features. However, none of these techniques can provide atomic accuracy while manufacturing devices with diverse functions, out of a wide range of materials.
Mechanosynthesis is the physical positioning and bonding of single atoms or molecules. Mechanosynthesis was first theorized to be possible in 1959 by theoretical physicist and Nobel laureate Richard P. Feynman, and was experimentally demonstrated in 2003 (Oyabu, N., Custance, O., et al. (2003). “Mechanical vertical manipulation of selected single atoms by soft nanoindentation using near contact atomic force microscopy.” Phys. Rev. Lett. 90(17)). Unlike the other manufacturing techniques discussed herein, mechanosynthesis offers the potential to create atomically-precise structures out of a wide variety of atoms or molecules, while being relatively unconstrained in the shapes and properties of the devices which can be built.
While several authors have explored the possibility of atomically-accurate manufacturing at varying levels of detail (Drexler, K. E. (1987). Engines of Creation: The Coming Era of Nanotechnology, Anchor; Drexler, K. E. (1992). Nanosystems: Molecular Machinery, Manufacturing, and Computation. New York, John Wiley & Sons; Freitas, R. (1999). Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience; Freitas, R. (2003). Nanomedicine, Vol. IIA: Biocompatibility, Landes Bioscience), many decades after it was first theorized, and more than 10 years after it was first experimentally demonstrated, mechanosynthesis remains a laboratory curiosity due to the number of engineering and design issues that need to be addressed in order to develop it into a commercially-viable manufacturing technology.
Mechanosynthesis and Mechanosynthesis Terminology. The present invention describes methods, systems and products relating to the manufacture of atomically-precise structures using atoms as raw material. These atoms are referred to as feedstock. The structures are referred to as workpieces. Workpieces are built using positionally-controlled tips, such as the tips on Atomic Force Microscopes, to move feedstock atoms into desired locations on a workpiece. Mechanical force is applied to atoms via these tips to make and break chemical bonds. This mechanical making or breaking of bonds at specific locations is called mechanosynthesis.
Note that other fields do use the term mechanosynthesis for different purposes. Most notably, some crystal and material synthesis processes refer to the use of milling or high pressure to create fine particles as “mechanosynthesis.” These techniques are not atomically-precise, and do not fall within our definition of the term.
The order in which atoms are added, removed, or altered during the course of building a workpiece is referred to as a build sequence or reaction sequence. A build sequence also encompasses the concept of a trajectory, which is the path along which an atom moves during a mechanosynthetic reaction. By using tips to move feedstock along a trajectory, to a specific location with respect to a workpiece, and then applying mechanical force to bond the atom into position, workpieces can be manufactured where the position of every atom is known.
Tips Used in Mechanosynthesis.
The mechanosynthesis processes described herein use a variety of ultra-sharp tips designed to move atoms with sub-angstrom precision and to facilitate different reactions with those atoms. The tips may be, but do not have to be, atomically-precise. While some embodiments of the invention use atomically-precise tips, others do not. For example, a bootstrap sequence is presented herein which allows the creation of atomically-precise tips using non-atomically-precise tips.
Atomically imprecise, but ultra-sharp tips, also called probes, are available commercially (e.g., from Nanotools Gmbh, Munich, Germany, or from NANOSENSORS, Neuchatel, Switzerland), or can be made using electron-beam induced deposition (EBID), among others techniques. Tay, A. B. H. and Thong, J. T. L. (2004). “Fabrication of super-sharp nanowire atomic force microscope using a field emission induced growth technique.” Review of Scientific Instruments 75(10). Such tips can serve as a starting point for the bootstrap process described herein.
In general, the important characteristic of a tip is that it reliably performs the desired mechanosynthetic reaction. Atomic precision is a helpful characteristic of tips for mechanosynthesis because knowing the precise placement of atoms on the tip allows design of reliable reactions via computational chemistry simulations. This is not to say that atomically imprecise tips could not be used in mechanosynthesis processes (as the bootstrap process discussed herein demonstrates), for example, by characterizing each tip before use, by designing reactions where variation at the tip does not substantially affect the intended reactions, or by designing procedures which result in minimal tip-to-tip variation when preparing tips. However, we focus on the use of atomically-precise tips (after bootstrapping) for a variety of reasons, including consistency and amenability to simulation.
Note that “tips” and “workpieces” are discussed extensively herein. However, while these terms are used for clarity, defining one structure as the tip and another as the workpiece can be arbitrary in some circumstances. Consider that, for example, when a tip removes a hydrogen atom from a workpiece, one might also say that the workpiece donated a hydrogen atom to the tip, logically reversing their roles. This distinction may seem pedantic, but it can be meaningful in some processes. For example, consider tip recharging or using one set of tips to build another set of tips. In such instances, because you are adding or removing atoms from a tip to recharge it for the next reaction, or because you are building new tips, a tip could be considered the workpiece.
Enabling Technologies.
Mechanosynthesis is largely based upon the confluence of atomic microscopy and computational chemistry. Microscopy techniques such as Scanning Probe Microscopy (SPM), Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have led to the ability to image and manipulate individual atoms, while computational chemistry has led to the ability to model structures which can be built by manipulating atoms, the reactions used to build those structures, and the tools required to carry out those reactions.
The ability to perform robust mechanosynthesis requires that one be able to position atoms (generally with sub-angstrom precision), that one be able to apply mechanical force to an atom in a specific direction to cause the making or breaking of bonds, that one be able to define a desired workpiece (or at least certain regions of the workpiece) with atomic precision, that one be able to calculate trajectories which will result in successful mechanosynthetic reactions and that one possess, or be able to design, tips to carry out the intended reactions. In addition to this list of necessities, it would be beneficial to be able to calculate the likelihood of pathological side reactions during mechanosynthetic reactions (the likelihood that, for example, a feedstock atom bonds to a workpiece atom adjacent to the intended target atom), the likelihood of pathological rearrangements before, during, or after a mechanosynthetic reaction, and to have control of the reaction environment (e.g., to make sure that it is inert and kept at an appropriate temperature).
AFM/SPM/STM Microscopy.
By 2006, sub-angstrom positioning in three dimensions was available for SPM. For comparison purposes, the diameter of a carbon atom is 1.54 angstroms, meaning that SPM tips could be reliably positioned to substantially less than the diameter of an atom. Also by 2006, such microscopy could be performed in ultra-high vacuum and at cryogenic temperatures, and “Vibration and drift have been controlled such that a probe tip can be held over a single molecule for hours of observation.” Bharat Bhushan (Ed.) (2006). Springer Handbook of Nanotechnology, Springer.
Subsequent advances in positional control have included MEMS-based platforms with additional degrees of freedom at sub-nanometer resolution which are not inherently tied to microscopy platforms (Yang, S. H., Kim, Y. -S., et al. (2012). “Microelectromechanical systems based Stewart platform with sub-nano resolution.” Appl. Phys. Lett. 101(6): 5.), and it should be noted that the invention discussed herein is not limited to being practiced with AFM, SPM or STM devices, but rather could use any device with the requisite positional control of a tip relative to a workpiece, and other requirements as may be necessary on a case-by-case basis (e.g., an inert environment and temperature control).
Note multi-tip SPM or related devices are well-known and may also be applied to the present invention. For example, force may be applied, or bonds formed, in more than one location simultaneously to stabilize an unstable intermediate workpiece structure during reactions.
Computational Chemistry in General
Computational chemistry algorithms have existed for decades, and it is well-known that if chemical reactions are simulated at a high enough level of detail, the results are extremely accurate (possibly more accurate than experimental results can be measured). Such simulations, for any large number of atoms, require substantial computer processing power. Jensen sums this up succinctly with the following quote:                “The only systems that can be solved exactly are those composed of only one or two particles . . . Numerical solutions to a given accuracy (which may be so high that the solutions are essentially “exact”) can be generated for many-body systems, by performing a very large number of mathematical operations.” Jensen, F. (2007). Introduction to Computational Chemistry, John Wiley & Sons.        
While the definition of “a very large number of mathematical operations” tends to change over time as computing technology progresses, generally such calculations require either supercomputers or other specialized computer hardware (e.g., ASIC or GPU), or clusters of commodity computer hardware. Processing power (based on, for example, ASIC, CPU, GPU, quantum computers, or other computational means) tends to be the limiting factor in such computations, although the memory and storage requirements (e.g., RAM, ROM, SSD, conventional hard drives, or other storage means) are not necessarily trivial.
It should be noted that there are many algorithms which can be used for computational chemistry, and that choices as to which algorithms, and when appropriate, what basis sets to use, must be made on a case by case basis considering the reactions and/or structures being analyzed, types of atoms, number of atoms, required accuracy, available computing power and what characteristics are of interest (e.g., an algorithm capable of analyzing, for example, the Young's Modulus of a given structure may be inappropriate for determining that same structure's infrared spectra or chemical reactivity). And, it may be appropriate to use multiple algorithms on the same molecular model (e.g., ONIOM). We describe herein the algorithms and basis sets that we have used to calculate reactions and build sequences, and simulate workpieces.
Computational Chemistry in Mechanosynthesis
Even on powerful computers, simulating large numbers of atoms at high levels of detail can be extremely computationally-demanding. However, an entire mechanosynthetic system need not be simulated at a high level of detail. Mechanosynthesis can be carried out in a more controlled environment than, for example, traditional liquid or gas phase chemistry, or biology, resulting in the ability to simplify simulations by reducing the number of atoms which are simulated at high levels of detail.
In mechanosynthesis, only a few positionally-controlled atoms are participating in a reaction at any given time. Most reactions away from the intended reaction position can be prevented by using an inert environment (e.g., a vacuum), and the ability to carry out reactions at low temperatures helps with reactions that cannot be prevented in this manner. Therefore, the number of atoms that are relevant to a given reaction and thus must be simulated at a high level of detail is quite small compared to the overall mechanosynthetic system or to other common settings in which chemical reactions take place. The result is that it is feasible to use computational chemistry techniques to simulate mechanosynthetic systems and reactions in a level of detail that enables one to make accurate predictions about the behavior of those systems and reactions.
Element Grouping and Simulation
When referring to groups of elements herein, we may talk about metals, non-metals, noble gases, transuranic elements, stable elements (which we define as non-radioactive isotopes and isotopes with half-lives long enough to support manufacturing and use of a product), or other logical groupings. The rationale behind these groupings would be obvious to one skilled in the art: generally the distinction is one of chemical properties (e.g., those in the same family on the periodic table or with the same valence), simulation feasibility, or practicality (e.g., safety aside, creating a device using isotopes with very short half-lives could pose problems in manufacturing and use of a device before the isotopes decay). In instances where a seemingly-arbitrary group of elements is specified, this is generally because we have simulated reactions using the elements in the group.